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Introduction

Source to Spectrum (S2S) is a new collaborative research project on economic mineral systems in South Australia funded under the Australian Research Council Linkage scheme. The universities of South Australia, Adelaide and Monash have partnered with the Geological Survey of South Australia, Fortescue Metals Group, Minotaur Exploration, Investigator Resources and Rex Minerals for the project. The South Australian and Commonwealth governments, via the Australian Research Council, have contributed over $0.5 million as well as in-kind support which has provided an average funding leveraging ratio greater than 1:7 for the industry partners.

This article focuses on the motivation behind the research project and the scientific aims and outputs that have the ultimate aim of facilitating mineral exploration success in South Australia.

The discovery of the Olympic Dam iron oxide – copper–gold (IOCG) – uranium – rare earth elements (REE) deposit in 1975 was a pivotal moment in South Australia’s economic history. Since that time, the sheer size and richness of the Olympic Dam deposit has resulted in a strong bias towards exploration for Olympic Dam style IOCG deposits within South Australia. This is reflected in the core inspection ‘heat map’ produced by Michael and Reid (2017), identifying the Stuart Shelf region as a primary focus of core inspections. The somewhat enigmatic nature of IOCG deposits as a deposit class, with Olympic Dam as the jewel in the crown, has also strongly influenced the research agenda and methodologies applied to South Australian deposits. S2S aims to redress that bias to better characterise the spectrum of deposits on the Gawler Craton.

Figure 1 Simplified geology of the Gawler Craton showing the extent of the c. 1590 Ma intrusives and volcanics of the Hiltaba–GRV felsic large igneous province and the two regions of investigation. Both regions have a range of exposed crustal levels and varying precursor geology. Age constraints are taken from Skirrow et al. (2007) and Reid et al. (2013), and demonstrate ~30 Myr of thermal drivers for fluid flow, alteration and mineralisation.

The c. 1595–1570 Ma Hiltaba Suite and Gawler Range Volcanics (GRV) magmatic event is the thermal system ultimately responsible for formation of the well-known IOCG deposits of Olympic Dam, Prominent Hill, Carrapateena and Hillside. In contrast to these deposits that have been discovered in the last 42 years1, some of the earliest deposits mined on the Gawler Craton represent a different end-member of the Hiltaba–GRV deposit spectrum. The central Gawler Craton has been described for the last ~20 years as a ‘gold-dominated’ province associated with the Hiltaba–GRV event (Fig. 1). Historical mining in the Tarcoola region has been augmented by recent identification of gold prospects such as Barns, Baggy Green, White Tank and Tunkillia. Notably, gold mining has recently recommenced at Tarcoola (WPG Resources 2017). Spread throughout the interface between the IOCG and gold-dominated provinces are a diverse series of prospects and deposits with enrichments in lead–zinc (Menninnie Dam and Telephone Dam), silver (Paris) and gold (Weednanna). This spectrum of deposits is extended by further variation within the IOCG-class, with early skarn alteration related IOCG (e.g. Hillside, Punt Hill, Emmie Bluff) complementing the ‘early magnetite’ alteration, hematite-breccia Olympic Dam style deposits. Recent exploration drilling by Minotaur Resources has also highlighted the potential for iron sulfide – copper–gold mineralisation (ISCG) in the Gawler Craton (Belperio 2016).

Geochronology findings suggest that the spectrum of deposits can all be temporally linked to the Hiltaba–GRV magmatic event (Fig. 1). Magmatism, metamorphism and mineralisation are distributed across a 30 Myr window with insufficient age resolution to assign individual alteration or deposit styles to specific geologic episodes within this interval. Precise temporal constraints relative to Hiltaba–GRV magmatism may be a useful tool for identifying genetically important magmatic sub-suites as it is widely suggested that magmatism is likely to have been a source of fluids and/or metals in the formation of each style of deposit. However, a second fluid is commonly invoked within models of IOCG formation to upgrade what would otherwise be low-grade or uneconomic mineralisation. In a similar fashion, the Menninnie Dam lead–zinc deposit has been interpreted to involve two fluid sources (magmatic and meteoric), with lead potentially sourced from a mixture of Hiltaba Suite magmatism and the surrounding metasedimentary country rock (Roache 1996). Deposits from the central Gawler gold province have not been conclusively identified as magmatic or orogenic-related, but some links to magmatism have been suggested for the Tarcoola deposit in particular (e.g. Fraser et al. 2007).

1 Historical workings adjacent to the modern Hillside deposit possibly make Hillside an exception to this timeline.

What causes the spectrum of mineral deposits? Linking prospectivity and preservation

In the last 40 years significant advances have been made towards understanding the formation of the South Australian IOCG deposits, with a lower volume of research focused on known lead–zinc, gold and silver mineralisation. S2S aims to produce a coherent framework that outlines the driving mechanisms behind each style of Hiltaba–GRV-aged mineralisation in the Gawler Craton. An overarching assumption to be tested is the importance of Hiltaba and GRV magmas as transporters of fluids and metals integral to deposit formation. An alternative genetic mechanism is that the Hiltaba–GRV magmatism produced at least some styles of mineralisation purely through the creation of high geothermal gradients in the crust leading to the generation and circulation of hydrothermal fluids through metamorphic processes. In almost all cases, the variation in mineralisation should be linked to either the composition or conditions of the crust within which the spectrum of deposits are found (e.g. Reid and Fabris 2015), and/or related to the type of magmatism in the hosting crustal column (e.g. Stuart Shelf versus central Gawler). Identifying the specific characteristics of each type of crustal column that contribute to the respective mineralisation styles will be a central outcome of S2S.

Genetic models for mineralisation have potentially limited relevance for improving exploration efficiency and outcomes within unexposed terrains such as the Gawler Craton. A key applied aim of S2S is to determine not only the likelihood of deposit formation in a particular crustal column, but also the likelihood of preservation after formation. As many of the deposits are likely to have formed in the upper crust, the potential for erosion is high. As an example, Oak Dam is interpreted to have been an ironstone inselberg at the time of Pandurra Formation deposition (Davidson et al. 2007) – erosion of the inselberg to the level of the surrounding plains would have removed the bulk of mineralisation. Determining the likelihood of preservation will involve an assessment of the pressure and temperature conditions associated with deposit formation (e.g. IOCGs, ISCGs, Pb–Zn, Au, Ag), which will be cross-referenced against the pressure and temperature conditions preserved before and after the timing window for mineralisation, and against basement topography. S2S aims to produce a series of metal-specific prospectivity maps, each taking into account the likelihood of deposit formation and preservation at the scale of prospective plutons and fault-bound blocks within the basement.